PULVERISED COMBUSTION
SYSTEM
 PULVERISATION
 FUELS
 COAL
 COAL APPLICATIONS
 COAL COMBUSTION
 COMBUSTION OF FUELS- SOLID, LIQUID & GAS
 EMERGING TRENDS
 Nox FORMATION AND CONTROL




Coal dust is a fine powdered form of coal
More surface area per unit weight than lumps of coal
More susceptible to spontaneous combustion
Pulverised coal has significant dust explosion hazard
FUELS
Solid Fuels
Coal
 Peat
 Lignite
 Bituminous
 Anthracite
Liquid Fuels
 HSD
 LFO
 HFO
Gaseous Fuels




BFG
Producer Gas
LPG
Natural Gas
Proximate
Analysis
Ultimate
Analysis
COAL
C
40-90%
H
2-7%
O
<18-30%
S
<6%
N
1-5%
Char
20-70%
Ash
5-45%
H2O
2-20%
VM
15-30%
 Heterogeneous organic fuel
formed mainly from decomposed
plant matter.
 Over 1200 coals have been
classified.
Coalification forms
different
Coal Types:
Peat
Lignite
Bituminous coal
Anthracite
Evolution
Time,
Pres.&Temp.
Coal Rank
 Home – heating and
cooking
 Transportation – steam
engines
 Industry – Iron & steel
 Electricity – power plants
COMBUSTION
Oxidation
oxygen combines with other elements and forms
oxidies.
Combustion,
 A special form of oxidation
 Oxygen combines with fuels; coal, oil, gas
 substantial amounts of heat is liberated.
The Degree of Flammability depends

convertibility to a gas,-nothing truly burns until it
is a gas.
 Nature of fuel
 Quantity of the fuel,
Stages of Combustion
 Ignition
 Combustion Stability
 Completion of Combustion
 Combustion process, ignition occurs in vapour
phase.
 Solid and liquid fuels get ignited from their
vapours.
Combustion Stability:
 Ignited flame shall be sustained further so that the process
of combustion would be continuous.
Completion of Combustion:
 Fuel staying for the minimum period (residence time)
 Completely oxidizing the combustible.
Considerations of combustion:
 Safety consideration during fuel handling
 Generation of Pollutants viz. Nox and Sox .
volatiles
homogeneous
combustion
CO2, H2O, …
coal particle
p-coal, d=30-70m
char
heterogeneous
combustion
CO2, H2O, …
devolatilization
tdevolatile=1-5ms
tvolatiles=50-100ms
tchar=1-2sec
t
Combustion of Fuels in furnace
 Preparing the fuel and air ;
Converting the complex fuel into elementary
fuels;
Right fuel and air mixture
Transferring heat from the products of
combustion to the boiler or other surfaces.
The physical processes influencing
pulverized coal combustion
 Turbulent/Swirling flow of air and coal.
 Turbulent/Convective/molecular diffusion of
gaseous reactants and products.
 Convective heat transfer through the gas and
between the gas and coal particles.
 Radiative heat transfer between the gas and
coal particles and between the coal/air
mixture and the furnace walls.
COMBUSTION OF DIFFERENT FUELS
Solid Fuel
 Volatile matter is released over a temp. of 250 - 900
deg. C.
 The volatile matter is first ignited.
 The coal particle upon releasing the volatile matter
become a char. The char slowly burns out.
Liquid Fuel
 Boils and releases volatile matter and gets ignited
 The balance char completes the combustion.
COAL - AIR BALANCING IN FUEL PIPING
Un balance in Coal- air flow into the furnace results
 uneven heat release
 uneven distribution of excess air
 unpredictable Nox formation
Remedies
 Selection of Orifices for Coal air two phase flow regime.
 Provision of on -line adjustment
dampers in coal air flow
path.
 On-line measurement of coal -air flow
using microwave
OIL FIRING SYSTEM FEATURES
Fuel Oil Preparation
 Pumping the oil and heating it are the major preparatory
functions.
 Filtration of oil to remove any dust, dirt, sediments, sledge
etc.
 This renders long trouble free service life to pumps,
valves, atomisers etc
 Maintaining the HFO temperature constant, corresponding
to the atomizing viscosity of 15 to 20 centistokes, is
essential for better fuel oil atomization.
 A lower temperature of fuel oil impairs the burner
performance and a higher temperature causes oil cracking.
Fuel Oil Atomisation
Atomisation
 spraying the fuel oil into fine mist
 Better mixing of the fuel with the combustion air
 Pressure and viscosity influence atomisation
 Pressure energy of the steam to velocity energy, which
breaks up the oil stream into fine particles
Poorly atomisation results in
 Bigger spray particles
 Longer burning time
 Carryovers of carbon and
 flame instability due to low rate of heat liberation
 Incomplete combustion and smoke
Oil Recirculation
 To warm up the oil supply lines
 To maintain correct atomising temperature
System Vents
 Fuel oil heaters
 Oil strainers
 oil & steam lines - get rid of air locks.
System Drains
 Oil lines with a drain valve at the lowest point.
BURNERS:
 To deliver coal , oil and air in a proper proportion
 To facilitate ignition energy to the coal air stream
 To sustain the ignition
 To provide a stable flame during the operation
Types of Burners
 Tangential Burners
 Wall Burners, Ex: Low Nox R burners
 Down shot or fan tail burners
Modern Burners are equipped with:
 Separate flame envelope ports for coal, oil and gas
 Secondary air control to adjust the flame envelops
 Ignitors
 Flame Scanners - detect the distinct flames in an
enclosure
 Flame Stabilisers
 Flame Analysers
Burner Arrangement
Tangential firing:
 Four tall windboxes (combustion air boxes) one at each
corner of the furnace.
 The oil and gas burners are located at different levels or
elevations of the windboxes.
 The coal , oil and gas burners are sandwiched between
air nozzles or air compartments.
 That is, air nozzles are arranged between gas spuds,
one below the bottom gas spud and one above the top
gas spud.
Burner Tilt:
 The burners are tiltable +/- 30o about horizontal,
 To shifts the flame zone across the furnace height
 To control over steam temperature
Combustion Air Distribution
The Combustion air
 Primary Air (PA) and Secondary Air (SA)
 Secondary Air(SA) provided from FD Fans
 Primary Air(PA) provided from PA Fans
Ignitors

Oil and gas are ignited by a pilot flame.
Type of Ignitors



Oil ignitor
Gas ignitor
High Energy Arc ( HEA) ignitor
Flame Sensing
Devices
Flame sensing devices are broadly grouped in to
 Infrared flame sensors
 UV flame scanners
 Visible light scanners
Recent development
 Flame analysers for multiple fuels
Emissions of Combustion - Pollutants
1. Nox emission
2. Sox emission
3.CO 2 emission (Green House Gases)
4. CO emission
5. Particulate emission
Emerging Trends
in
combustion system Design
Emerging trends in combustion system Design
 Multiple fuel Burners
 Low Emission Burner
- Technology development
 Longer guarantee period for high ash coals
- Material selection,
- Improved design features
 New devices such as thermal analysers.,
Emerging trends in combustion system Design
Cont.……
 Micro processor based on line measurement and control
 Computer simulations using software tools viz., ANSYS, CFD.
 Virtual assembly using CAD tools
NOx Formation
and
Control Strategies
NOX FORMATION
 Thermal NOx Formation
 Nox Formation from Fuel Nitrogen
NOx Control options
Control Technique
NOx Reduction Potential(%)
Over fire air (OFA)
20-30
Low Nox Burners (LNB)
35-55
LNB + OFA
40-60
Re-burn
50-60
SNCR ( Selective Non
Catalytic Reduction)
30-60
SCR (Selective Catalytic
Reduction)
LNB with SCR
75-85
LNB with OFA and SCR
85-95
50-80
Selective Catalytic Reduction (SCR) Reactions
4 NO  4 NH 3  O2 
 4 N 2  6 H 2O
TiO2 or V2 O5 supported catalyst
or V2O5 supported catalyst
2 NO2  4 NH 3  O2 TiO
2

 3N 2  6 H 2O
AVAILABLE TECHNIQUES FOR REDUCING NOX FROM TANGENTIAL COAL FIRED UNITS
90
SCR
% NOX REDUCTION FROM BASE *
80
PM+
NH3/UREA
70
PM
60
CCTFS
50
LNCFS
W/OFA
40
30
CLOSE
COUPLED
OFA
20
10
0
INCREASING COST
* BASE IS A TYPICAL PRE 1970 TANGENTIALLY FIRED UNIT. BASE NOX
OVER FIRE AIR (OFA)
 Through additional air compartments, to handle 15
percent of total wind box air flow.
 Inhibits formation of both fuel Nox and thermal
Nox as an Oxygen Deficient environment is
established in the primary combustion zone
 20 to 30% reduction in Nox formation
NOX DEPENDENCE ON OVERFIRE AIR FLOW
350
300
NO (PPM @ 3% O2)
250
OFA VS NO
200
150
EXCESS O2 (%) VS NO
100
50
0
0
2
4
6
8
OVERFIRE AIR(%)
10
12
14
16
 While Nox
emission decrease linearly with increasing
over fire air, Excess air rises (i.e., More Air Is Needed To
Complete The Combustion).
 If over fire air is increased beyond 15%. This decreases
boiler efficiency due to the heating of extra air
Nox Reduction Techniques
STAGED COMBUSTION:
 Fuel Bound Nitrogen to be regulated
 High-Temperature Formation must be curtailed.
 Withholding of Some O2 from primary flame zone
 Air staging
LOW NOx BURNERs
Wall burners (oil and gas firing )
 Low NOx R-burner is capable of emitting NOx at a level of 150
ppm on oil firing.
Tangential firing ( Coal firing)
 CCOFA(Close Coupled Over Fire Air) feature.
 Separate Over Fire Air( SOFA ) .
THANK YOU ALL